revert "suart: fix input capture interrupt handling bug"

this reverts the previous commit d3d20bf3.

that fix was wrong -- if input capture is in use elsewhere (as
it is in the autoblind project, in ir.c), then we don't want
to clear its enable here.
suart: disable ICIE when start bit is received

as alex cornejo pointed out, without disabling this enable bit we
could never receive a full byte.  but only touch the bit if it's
in use by this module.  (untested)